The present invention relates to a process for photocatalytic removal of organic substances from waste waters.
Pyrogenic titanium oxide (commercially available as Degussa TiO2 P 25) is distinguished by its variety of possible applications in the field of photocatalysis as shown in the following sources:
(R. W. Matthews, S. R. McEvoy, J. Photochem. Photobiol.A: Chem., 64 (1992) 231-246;
R. I. Bickley et.al., Journal of Solid State Chemistry, 92 (1991), 178-190;
R. Franke, C. Franke, Chemosphere, Vol. 39, No. 15 (1999), 2651-2659; and
H. Zen, JETI (1998), 46 (10), 66-67).
It is used as reference material with high photocatalytic activity; see (V. Loddo et.al., Applied Catalysis B: Environmental 20 (1999), 29 -45.)
The present invention provides a process for photocatalytic removal of organic substances from waste waters, said process being characterized in that by way of the photocatalyst use is made of granulates based on pyrogenically prepared titanium dioxide having the following physicochemical characteristic data:
The granulate that is capable of being used in accordance with the invention can be prepared by pyrogenically prepared titanium dioxide being dispersed in water and spray-dried and by the granulates obtained being optionally tempered at a temperature from 150 to 1,100xc2x0 C. for a period from 1 to 8 h.
The pyrogenic titanium dioxide can be prepared from TiCl4 by a known method by means of high-temperature or flame hydrolysis (Ullmanns Enzyklopxc3xa4die der technischen Chemie, 4th Edition, Volume 21, page 464 (1982)).
The dispersion in water for preparing the granulates can exhibit a concentration of titanium dioxide from 3 to 25 wt. %.
Organic auxiliary substances may be added to the dispersion in order to enhance the stability of the dispersion and to improve the particle morphology after spray drying.
For example, the following auxiliary substances may be employed:
polyalcohols, polyethers, surfactants based on fluorohydrocarbons, alcohols.
Spray drying can be carried out at a temperature from 200 to 600xc2x0 C. Spinning-disc atomizers or nozzle atomizers may be employed in this process.
Tempering of the granulates can be carried out both in a stationary bed, such as in chamber kilns for example, and in an agitated bed, such as in rotary-tube dryers for example.
By varying the feed materials and the conditions in the course of spraying and tempering, the physicochemical parameters of the granulates, such as the specific surface area, the particle-size distribution, the compacted bulk density and the pH value, can be varied within the specified limits.
No additional auxiliary substances are required for the granulation. In comparison with non-spray-dried titanium dioxide, which has no defined agglomerate size, the titanium-dioxide granulate according to the invention has a defined particle size.
The titanium-dioxide granulate according to the invention enables dust-free handling. By reason of the high compacted bulk density, less packaging effort is required for transportation.